California’s clean-air agency voted on Friday to push ahead with stricter emissions standards for cars and trucks, setting up a potential legal battle with the Trump administration over the state’s plan to reduce planet-warming gases.

The vote, by the California Air Resources Board, is the boldest indication yet of California’s plan to stand up to President Trump’s agenda. Leading politicians in the state, from the governor down to many mayors, have promised to lead the resistance to Mr. Trump’s policies.

Mr. Trump, backing industry over environmental concerns, said easing emissions rules would help stimulate auto manufacturing. He vowed last week to loosen the regulations. Automakers are aggressively pursuing those changes after years of supporting stricter standards.

But California can write its own standards because of a longstanding waiver granted under the Clean Air Act, giving the state — the country’s biggest auto market — major sway over the auto industry. Twelve other states, including New York and Pennsylvania, as well as Washington, D.C., follow California’s standards, a coalition that covers more than 130 million residents and more than a third of the vehicle market in the United States.

“All of the evidence — call it science, call it economics — shows that if anything, these standards should be even more aggressive,” said the board member Daniel Sperling, a transportation expert at the University of California, Davis.

The board’s chairwoman, Mary D. Nichols, an assistant administrator at the Environmental Protection Agency under President Bill Clinton, was even more pointed, admonishing automakers for milking Mr. Trump for favors.

“What were you thinking when you threw yourselves upon the mercy of the Trump administration to try to solve your problems?” she asked. “Let’s take action today, and let’s move on.”

Long a forerunner in environmental regulation, California worked with the Obama administration on joint standards that became a crucial part of the country’s effort to combat climate change. Officials said the regulations would reduce the country’s oil consumption by 12 billion barrels and eliminate six billion metric tons of carbon dioxide pollution over the lifetime of the cars affected. That amounts to more than a year’s worth of America’s carbon emissions.

Adopted in 2012, the standards would require automakers to nearly double the average fuel economy of new cars and trucks by 2025, to 54.5 miles per gallon, forcing automakers to speed development of highly fuel-efficient vehicles, including hybrid and electric cars. Mr. Trump intends to lower that target.

Friday’s unanimous vote by the 14-member board, which affirmed the higher standards through 2025, amounted to a public rejection of Mr. Trump’s plans.

Now, the question is how — or whether — the Trump administration will handle California’s dissent. The administration could choose to revoke California’s waiver, at which point experts expect the state would sue.

California sued the George W. Bush administration after it challenged California’s waiver in 2007. Mr. Obama reversed the federal challenge.

The White House and the E.P.A., which have not yet determined their plans for the California waiver, did not immediately respond to a request for comment.

Several states that follow California’s rules raced to its defense. “We’ve come a long way together,” said Steven Flint, director of the air resources division of the New York Department of Environmental Conservation. “We’re with you, and we believe in what you’re doing.”

Environmentalists and public health experts have criticized the automakers’ resistance to emissions rules under the Trump administration as an about-face. All major automakers previously voiced support for the more stringent standards.

After the election of Mr. Trump, a group representing the nation’s biggest makers of cars and light trucks urged a reassessment of the emissions rules, which the group said posed a “substantial challenge” for the auto industry.

Automakers now complain about the steep technical challenge that the stringent standards pose. They have estimated that only about 3.5 percent of new vehicles are able to reach it, and that their industry would have to spend a “staggering” $200 billion by 2025 to comply.

A separate study by the International Council on Clean Transportation, a think tank supporting emissions controls, has estimated that the cost of meeting those standards could be overstated by as much as 40 percent. And auto industry experts have warned that a slowdown in America’s shift toward efficient cars could leave its auto market a global laggard.

John Bozzella, chief executive of Global Automakers, an industry trade group, said before the California vote that companies agreed on the need to continue to reduce greenhouse gas emissions and improve fuel economy. But he urged California to fall into line with federal rules.

“There is a more effective way forward than regulatory systems that are different,” Mr. Bozzella said. He also suggested that demand for clean cars remained relatively tiny.

What was required, he said, were standards that “balance innovation, compliance and consumer needs and wants.”

Automakers have also been critical of a California’s zero-emission vehicle program, which requires automakers to sell a certain percentage of electric cars and trucks in California and nine other states. The board voted on Friday to continue that program.

Politicians in California, one of the country’s most Democratic states, have embraced acting as a bulwark against Mr. Trump’s policies, promising to defend the state’s laws on immigration, health care and the environment. Many cities in California have broad “sanctuary” policies aimed at protecting the rights of undocumented immigrants. State law also provides some protections for immigrants from being turned over to federal authorities for deportation.

In addition, Gov. Jerry Brown, a Democrat, declared that California would continue to work toward its legally required target of reducing carbon emissions to 40 percent below 1990 levels by 2030. And the state has retained Eric H. Holder Jr., the former United States attorney general, to advise on potential legal fights with the White House.

Even at the federal level, the president’s announcement alone will not be enough to immediately roll back emissions standards, a process expected to take more than a year of legal and regulatory reviews by the E.P.A. and the Transportation Department. The Trump administration would then need to propose its own replacement fuel-economy standards.

Still, the Trump administration’s move to ease emissions rules is the first part of an expected assault on Mr. Obama’s environmental legacy. In the coming weeks, Mr. Trump is also expected to announce that he will direct the E.P.A. to dismantle Obama-era regulations on pollution from coal-fired power plants.

The E.P.A. administrator, Scott Pruitt, has said he does not think carbon dioxide is a primary cause of global warming, a statement at odds with the scientific consensus on climate change.

Bonnie Holmes-Gen of the American Lung Association of California, one of many health and environmental groups that spoke at the board meeting, said moving away from strict emissions standards would hurt public health and the health of the planet. She urged the state to stay its course.

“The public is bearing a huge cost — billions of dollars in health expenses and damage from climate,” Ms. Holmes-Gen said. “I urge California to keep us on track.”

Correction: March 25, 2017
An earlier version of this article misstated Steven Flint’s position. He is director of the air resources division of the New York Department of Environmental Conservation, not the director of the department.